Abnormal sex determination leading to sex reversal occurs in approximately 1 in 15,000 births; the most frequently observed cases involve XY females (Dewing et al., 2002). The fact that the majority of human sex reversal cases remain unexplained requires a better understanding of all the genes involved in gonadal sex determination and differentiation. The proposed experiments are designed to test the hypothesis that genes expressed in somatic support cells (SSCs) of XX and XY embryonic gonads play an important role in mammalian gonadal sex determination and differentiation. Aims of this proposal are: (1) Identify genes differentially expressed in SSCs of XY and XX embryonic mouse gonads using microarray analysis. SSCs will be isolated by flow cytometry from undifferentiated and differentiated embryonic gonads of transgenic mice containing an EGFP reporter construct expressed specifically in the gonadal SSCs. (2) Identify the autosomal modifier loci involved in B6 FOG2 XY sex reversal. FOG2 is a nuclear protein expressed in SSCs. A Fog2 null allele (Fog2-) causes ovarian development in C57BL/6J (B6) but not in DBA/2J (D2) XY mice. Genome-wide linkage analysis will be performed using single nucleotide polymorphic (SNP) markers. Chromosomal regions involved in B6 FOG2 XY sex reversal will be compared to previously identified B6 autosomal loci involved in other XY sex reversal models. The proposed experiments will identify genes expressed in gonadal SSCs that play a role in gonadal XY sex reversal.